The present invention relates to a method and reagents for determining ligands in biological fluids such as serum, plasma, spinal fluid, amnionic fluid and urine. In particular, the present invention relates to a fluorescence polarization immunoassay procedure and to tracers employed as reagents in such procedures. The fluorescence polarization immunoassay procedure of the present invention combines the specificity of an immunoassay with the speed and convenience of fluorescence polarization techniques to provide a means for determining the amount of a specific ligand present in a sample.
Competitive binding immunoassays for measuring ligands are based on the competition between a ligand in a test sample and a labeled reagent, referred to as a tracer, for a limited number of receptor binding sites on antibodies specific to the ligand and tracer. The concentration of ligand in the sample determines the amount of tracer that will specifically bind to an antibody. The amount of tracer-antibody conjugate produced may be quantitively measured and is inversely proportional to the quantity of ligand in the test sample.
In general, fluorescence polarization techniques are based on the principle that a fluorescent labeled compound when excited by linearly polarized light will emit fluorescence having a degree of polarization inversely related to its rate of rotation. Therefore, when a molecule such as a tracer antibody conjugate having a fluorescent label is excited with linearly polarized light, the emitted light remains highly polarized because the fluorophore is constrained from rotating between the time light is absorbed and emitted. When a "free" tracer compound (i.e., unbound to an antibody) is excited by linearly polarized light, its rotation is much faster than the corresponding tracer-antibody conjugate and the molecules are more randomly oriented, therefore, the emitted light is depolarized. Thus, fluorescence polarization provides a quantitive means for measuring the amount of tracer-antibody conjugate produced in a competitive binding immunoassay. Various fluorescent labeled compounds are known in the art. U.S. Pat. No. 3,998,943 describes the preparation of a fluorescently labeled insulin derivative using fluorescein isothiocyanate (FITC) as the fluorescent label and a fluorescently labeled morphine derivative using -aminofluorescein hydrochloride as the fluorescent label. Carboxyfluorescein has also been used for analytical determinations. R. F. Chen, Anal. Lett., 10, 787 (1977) describes the use of carboxyfluorescein to indicate the activity of phospholipase. However, carboxyfluorescein is not conjugated according to the present invention. It is encapsulated in lecithin liposomes, and it will fluoresce only when released by the hydrolysis of lecithin.